1. Field of the Invention
The present invention relates to a synthetic aperture radar system utilizing the Doppler effect caused by the movement of a radar platform to improve the cross-range resolution. The present invention particularly concerns improvements in the capacity of such a synthetic aperture radar system for detecting a moving object.
2. Description of the Related Art
FIG. 4 is a block diagram of a synthetic aperture radar system disclosed in Donald R. Wehner, "High Resolution radar", Artech House, Section 6. More particularly, this synthetic aperture radar system is constructed by modifying such a structure as described in the above publication on page 260 and shown in FIG. 6.41, according to the description of the same publication.
The synthetic aperture radar system is adapted to detect an object by the use of a single antenna beam and includes means for transmitting radio waves. The transmitting means comprises a transmitter 1, a circulator 2 and an antenna 3. The transmitter 1 generates radio waves modulated by pulse signals. The circulator 2 functions as a transmission/reception changeover circuit for supplying the output of the transmitter 1 to the antenna 3 and for supplying the output of the antenna 3 to a receiver 4. When the antenna 3 receives the radio waves from the transmitter 1 through the circulator 2, the antenna 3 radiates the radio waves. Since the radio waves have been modulated by the pulse signals, the synthetic aperture radar system will radiate the radio waves with a repetition interval determined by the pulse signals.
The radio waves radiated from the antenna 3 are reflected by the objects, earth, sea and so on. The reflected radio waves, that is, echoes are inputted in the receiver 4 through the circulator 2. The receiver 4 amplifies the received radio waves before they are subjected to phase detection.
On the results from the phase detection, the receiver 4 generates a two-dimensional digital output signal which is represented by a range bin number m and a pulse hit number n. The range bin numbers are applied to the respective transmission timings while the pulse hit numbers represent the positions of the objects and others relating to the creation of the echoes in the reflection. In other words, each of the range bin numbers can specify a transmission timing or azimuth while each of the pulse hit numbers determines a range between the synthetic aperture radar system and an object or the like.
The post-stage of the receiver 4 is connected to a pulse compression unit 5 which performs a pulse compression to the two-dimensional digital signals from the receiver 4 on their correlation along the direction of range bin. Such a process improves the range resolution in the synthetic aperture radar system.
The post-stage of the pulse compression unit 5 includes a circuit for improving the cross-range resolution of the synthetic aperture radar system. This circuit comprises Fourier transform units 6A and 6B, a reference signal generator 7, a complex multiplication unit 8 and an inverse Fourier transform unit 9.
One of the Fourier transform units 6A Fourier-transforms the output of the pulse compression unit 5 for the pulse hit numbers n. The other Fourier transform unit 6B Fourier-transforms the output of the reference signal generator 7. The complex multiplication unit 8 multiplies the output of the Fourier transform unit 6A by the output of the Fourier-transform unit 6B to form complex data. The complex data is then inversely Fourier-transformed by the inverse Fourier transform unit 9. Thus, the cross-range resolution can be improved.
After the signals have been improved in range resolution and cross-range resolution, they are then provided to a square-law detection unit 10. The signals are subjected to square-law detection in the square-law detection unit 10. The square-law detection determines an electric power corresponding to each pixel in the screen of a display unit 11. The square-law detection unit 10 outputs the results to the display unit 11. As a result, the screen of the display unit 11 displays radar images representing the positions, distances, azimuths and the like of the object and others around the radar system.
In such an arrangement, the range resolution is determined by the band width of transmitted pulse signals. The cross-range resolution is determined as follows:
It is now assumed that the radar system is mounted in a moving platform such as aircraft or the like. It is further assumed that this moving platform moves straight at a velocity V as shown in FIG. 5 and that radio waves are radiated in a direction substantially perpendicular to the direction of movement of the platform. It is still further assumed that the transmitted radio waves are reflected by a stationary object such as the ground.
In such a case, the distance R(t) between the moving platform and the object can be represented by the following equation (1): EQU R(t)=R.sub.0 -Vt cos .theta..sub.0 +V.sup.2 sin.sup.2 .theta..sub.0 t.sup.2 / (2R.sub.0) (1)
where R.sub.0 is equal to R(0) and .theta..sub.0 is an expected angle of the object at time t=0, which angle is a reference in the direction of advance.
When the moving platform moves relative to the stationary object, Doppler effect is created. An instantaneous value in the Doppler frequency, that is, instantaneous Doppler frequency f.sub.d (t) is represented by the use of a transmission wavelength .lambda. from an equation (2). If the synthetic aperture time is T, the band width B of the Doppler frequency and the cross-range resolution .DELTA.r are represented by equations (3) and (4), respectively. EQU f.sub.d (t)=2/.lambda.(V cos .theta..sub.0 -V.sup.2 sin.sup.2 .theta..sub.0 t/R.sub.0) (2) EQU B=f.sub.d (-T/2)-f.sub.d (T/2) (3) EQU .DELTA.r=V sin .theta..sub.0 /B=.lambda.R.sub.0 /2VT sin .theta..sub.0( 4)
A synthetic aperture radar for observing a continuous field of view has the maximum resolution when the value .theta..sub.0 is equal to 90 [deg]. In the normal operation, such a setting is selected. Since the synthetic aperture time T is given by an antenna beam width .theta..sub.B from an equation (5), the band width B of the Doppler frequency and the cross-range resolution .DELTA.r are given by equations (6) and (7): EQU T=R.sub.0 .theta..sub.B /V (5) EQU B=2V.theta..sub.B /.lambda.=V/.DELTA.r (6) EQU .DELTA.r=.lambda./2.theta..sub.B =B/B (7)
When such a synthetic aperture radar is to be used to detect and image an object moving on the ground or sea, the band width of the Doppler frequency in the radar echo from the moving object is consistent with the reflected waves from the stationary object, but different in center frequency from that of the reflected waves. If the spectrums of the radar echo from the stationary and moving objects can be separated from each other in the frequency domain as shown in FIG. 6, the moving object can easily be detected and imaged. On the contrary, if the two spectrums are over-lapped on each other as shown in FIG. 7, it is difficult to detect and image the moving object. The overlap of the two spectrums is created when the velocity of the moving object is too low. In order to separate the spectrums from each other, it is therefore required to reduce the band width B of the Doppler frequency. Alternatively, it is required to increase the velocity of the moving object to be detected and to abandon the detection of an object moving at lower velocities. If the band width B of the Doppler frequency is decreased, it is noted that the cross-range resolution .DELTA.r is degraded as will be apparent from the equation (7).